This invention relates to automatic shifting power transmissions and more particularly to power transmissions having a single torque transmitter operable to selectively actuated to complete both forward and reverse drives.
In automatic shifting power transmissions it is common practice to install a hydrodynamic fluid drive, such as a torque converter, between the power source (engine) and a multi-speed gear configuration, such as a planetary gear arrangement. The torque converter (TC) provides a torque multiplier between the engine and the gearing to improve the vehicle launch performance. The torque ratio of the TC is generally in the range of 1.60 to 3.3 depending on the particular application. As is well-known, the TC is a slipping device that has a high efficiency loss at vehicle launch. This loss decreases, but continues, as the TC approaches a 1.0 to 1.0 speed ratio at high speed and low torque. In recent times, a torque converter clutch has been added to most transmissions to effectively remove the TC from the power path and thereby improve the overall efficiency of the transmission.
Other considerations have been given to improving the overall efficiency of the transmission. For example, the use of a starting clutch in lieu of a TC has been suggested and in some instances utilized. The advent of electronic controls improves the operation of a starting clutch as a vehicle launch device. The clutch is, however, a rotating device with all of the complexities associated with such a device. The control needs considerable accuracy to insure consistent fill times, and to compensate for variable fluid leaks at the rotating shaft seals. This requires accurate hydraulic flow volumes and pressure control over a wide range of operating requirements. Also the use of a starting clutch merely replaces one rotating mechanism with another, albeit a more efficient mechanism. There is only slight axial space saving and perhaps more complex control features.
It is an object of the present invention to provide a power transmission with an improved friction launch mechanism.
In one aspect of the present invention, a selectively operable torque transmitting mechanism is engaged to initiate both forward and reverse operation in a power transmission. In another aspect of the present invention, the torque transmitting mechanism controls the torque transmitting operation of one planetary gear member of a ratio planetary gear set in a multi-speed power transmission. In yet another aspect of the present invention, the planetary gear member has the same directional sense during launch in both the forward and reverse operation.
In still another aspect of the present invention, the torque transmitting mechanism is a stationary reaction member. In a further aspect of the present invention, the torque transmitting mechanism does not apply an unbalanced radial force to the planetary gear member. In yet a further aspect of the present invention, the torque transmitting mechanism utilizes half wrap bands with a double servo to reduce the radial forces and decrease the sensitivity of the torque transmitting mechanism to changes in the coefficient of friction.